It is thought that the proper function of all voluntary motor systems depends on the establishment of highly ordered connections between motoneurons and muscles. The long-term objective of proposed research is to identify mechanisms involved in the formation of specific neuromuscular connections during development The embryonic avian spinal cord and hindlimb will be used as a model system. Previous studies have shown that limb motoneurons are specified or have a target identity prior to axon outgrowth and that they recognize specific cues in the periphery which guide them to correct targets. Experimental evidence also suggests that these cues originate from the LS somatopleural mesoderm, the precursor of all limb connective tissues. Proposed experiments will characterize the role of connective tissue precursor cells in guiding motoneuron axons in the limb. Studies will define the organization of these cells during axon outgrowth stages and elucidate how and when guidance cues become specified within limb connective tissues. Other experiments 1) will examine the role of Schwann cells in limb motoneuron axon outgrowth and 2) will address the question of how a motoneuron acquires a target identity prior to axon outgrowth. The methods to be used include embryonic surgical manipulations and interspecies grafting, anatomical and electrophysiological techniques for assessing the specificity of motoneuron projections, immunohistochemistry, and explant co- culture. These studies will provide new insights into mechanisms of motoneuron and target cell specification and motoneuron axon guidance. Additionally, they may suggest methods of enhancing axon outgrowth following injury.